1. Technical Field
The present disclosure generally relates to microstructures, such as electrochemical devices in the form of transistors, and the like, in which conductive parts thereof may at least partially be formed from conductive polymer materials.
2. Description of the Related Art
Over the last decades polymer materials have been used in a wide variety of applications in many technical fields due to the fact that these materials may be synthesized at low cost, while the material characteristics can be adjusted in a wide range so as to address specific technical demands. Generally, a polymer material is comprised of large molecules, which in turn consist of sub-groups of reduced size, called monomers, which undergo polymerisation upon establishing appropriate process conditions.
Since polymer materials may also be produced so as to exhibit a wide range of electrical characteristics, for instance having a very low conductivity corresponding to an electrical insulator, up to moderately high conductivity values, polymer materials have become well established materials for various electronic devices, such as light emitting diodes, electrode structures, transistors, and the like, thereby taking advantage of conductive and insulating characteristics of the different types of polymer materials used. For example, in recent approaches the electrochemical behavior of a wide class of polymer materials is taken advantage of when forming electrochemical devices in the form of light emitting electrochemical cells, display devices, batteries and in particular electrochemical transistors. A very promising technical field in this respect is the manufacturing of sophisticated low-cost sensor structures, which may be implemented on the basis of polymer materials. In electrochemical devices both electrons and ions serve as charge carriers when the polymer material is subjected to a reversible electrochemical reduction and oxidation process. For example, in an electrochemical transistor an impedance modulation is utilized by switching between different reduction/oxidation states of the active transistor material. In this manner, a high impedance state and a state of moderately high conductivity may be established in a controlled manner.
Irrespective of the charge carrier mechanism in the conductive polymer material, an appropriate layer structure is provided in order to implement the desired function on the basis of a conductive polymer material. Although generally electrochemical electronic devices, such as transistors, are significantly less sensitive to a variation in lateral size, for instance compared to “regular” semiconductor-based field effect transistors, nevertheless a precise patterning of the conductive polymer material is desired, in particular, when electronic devices or any other components have to be provided in the form of microstructure devices, which are to be understood as devices, in which at least one lateral dimension is several hundred micrometers and significantly less. Consequently, it has been proposed to apply well-established manufacturing techniques, in particular optical lithography techniques, in order to realize microstructure devices on the basis of conductive polymer materials, wherein, however, the characteristics of the conductive polymer material under consideration may have a significant influence on the applicability of these well-established patterning techniques in the context of a specific polymer material.
For example, poly(3,4-ethylenedioxythiophene) (PEDOT) is a well-established polymer-based material, which in combination with an appropriate concentration of poly(styrene sulphonate) (PSS) is frequently used as a conductive polymer material known as PEDOT:PSS. This material is an aqueous dispersion and has significant advantages in terms of chemical stability, moreover it can be easily processed from solution into thin films that are stable in a wide pH range. Furthermore, PEDOT:PSS is a commercially available conductive polymer blend that provides for very high conductivity compared to other conductive polymer materials, wherein the conductivity may be adjusted by incorporating appropriate components, such as Ethylene Glycol, 2-nitroethanol, Dimethylsulfoxide and the like. Furthermore, PEDOT:PSS may be applied as a thin continuous layer by well-established deposition techniques, such as spin coating and the like.
Since PEDOT:PSS has many advantageous characteristics for forming microstructures, for instance in the form of electrochemical devices, great efforts are being made in developing patterning strategies, in which feature sizes of critical dimensions in the range of several hundred micrometers and less may be realized on the basis of well established lithography techniques preventing a substantially “direct” patterning of the conductive polymer materials in order to avoid undue loss of conductivity, which may frequently be associated with a photolithographic patterning process of these materials. On the other hand PEDOT:PSS is soluble in water-based developers used in photolithography and is also soluble in alcoholic solutions, which are frequently used to remove the photoresist after the patterning process.
Complex patterning strategies have been recently developed, in which a sacrificial material layer is applied in order to protect the sensitive polymer material upon patterning a photo resist layer, which in turn may be used to transfer the desired micrometer pattern into the conductive polymer layer. For example, Parylene is an appropriate sacrificial material, which in combination with appropriate resist liftoff techniques may be used in order to obtain a desired layout of the conductive polymer material. To this end, however, a plurality of deposition processes in combination with the actual lithography process is utilized, followed by a complex liftoff process possibly in combination with an intermediate resist removal process. This complex overall process strategy significantly contributes to overall production costs, thereby offsetting many of the advantages offered by the cost effective conductive polymer material.
In other approaches a stack of sacrificial layers is patterned so as to include a desired pattern, wherein subsequently a thin layer of the conductive polymer material is deposited. Thereafter, the actual patterning of the conductive polymer materials is accomplished by peeling off the sacrificial layer stack, thereby also removing unwanted portion of the conductive polymer material.
In other approaches the conductive polymer material may directly be patterned on the basis of imprint techniques, which also benefits from significant effort in terms of providing appropriate templates and process strategies, while any dielectric materials may still have to be provided in additional process steps. In other approaches Inkjet printing techniques may be used to directly pattern the conductive polymer material, but these techniques demand extreme control of deposition parameters.
The above-described conventional process strategies for patterning a conductive polymer material allow the fabrication of microstructure devices, however, significant effort in terms of additional process steps and sacrificial materials is needed in order to comply with the characteristics of the conductive polymer materials with respect to a possible interaction with chemical solutions that are typically used in well-established photolithography processes.